The demand for increased capacity in the area covering communication networks can be solved by the introduction of array antennas. These antennas are arrays of radiating elements that can create one or more narrow beams in the azimuth plane. A narrow beam is directed or selected towards the client of interest, which leads to a reduced interference in the network and thereby increased capacity. In U.S. Pat. No. 6,509,881 an interleaved single aperture simultaneous Rx/Tx antenna is disclosed.
A number of simultaneous fixed scanned beams may be generated in the azimuth plane by means of a Butler matrix connected to the antenna columns. The antenna element spacing is determined by the maximum scan angle as the creation of interference lobes due to repeated constructive adding of the phases (also referred to as grating lobes) must be considered. In order to scan a phased array antenna, the element positions must be small enough to avoid grating lobes. For an element distance of 1λ the grating lobe will appear at the edge of the visible space (non-scanning condition). If the beam then is scanned off boresight, the grating beam will move into the visible space.
Thus, a problem in designing antennas is that the radiating elements in an array antenna have to be spaced less than one wavelength apart in order not to generate troublesome grating (secondary) lobes and in the case of a scanned beam, the spacing has to be further reduced. In the limit case when the main beam is scanned to very large angles (as in the case of an adaptive antenna for mobile communications base stations), the element separation needs to be reduced to half a wavelength or less to avoid generation of grating lobes within visible space. Thus it can as a general rule be established that an antenna array with a fixed lobe should normally have an element distance of less than 1 wavelength while an antenna array with a scanable lobe should normally have an element distance of less than half a wavelength for obtaining a proper scanning angle range.
As disclosed in U.S. Pat. No. 6,351,243, radiating elements in an array antenna are often placed in a regular rectangular grid as illustrated in FIG. 1. The element spacing is denoted dx along the x-axis and dy along the y-axis. The beam directions are found by transforming from element space to beam space. The corresponding beam space for the antenna illustrated in FIG. 1 is found in FIG. 2.
In this case the main beam is pointing in the direction along the antenna normal. The beams outside the visible space (i.e. outside the unit circle) constitute grating lobes and they do not appear in visible space as long as the beam is not scanned and the element spacing is less than one wavelength along both axes (λ/dx>1 and λ/dy>1). For a large array, the number of radiating elements in the rectangular arranged grid is approximately given by NR=A/(dxdy), where A is the area of the antenna aperture.
When the main beam is scanned along the x-axis, all beams in beam space move in the positive direction by an amount, which equals a function expressed as sinus of the scan (radiating) angle. For each horizontal row in a one-dimensional scan in the x-direction we can express secondary maxima or grating lobes as
            x      m        =                  sin        ⁡                  (                      θ            s                    )                    +              m        ·                  λ                      d            x                                ,      m    =          ±      1        ,      ±    2    ,wherein xm is the position of lobe m, θs is the scan angle relative to the normal of the array and dx is the distance between the elements in the horizontal plane. As the distance between lobes here is λ/dx it will be realised that the largest element distance for a scan angle producing no grating lobes within the visible region is
      d    λ    <      1          1      +              sin        ⁢                                  ⁢                  (                      θ            max                    )                    
In a case illustrated in FIG. 3, a second beam (grating lobe) enters visible space in addition to the main beam. This may be avoided by reducing the element spacing along the x-axis. When the element spacing is less than half a wavelength (i.e. λ/dx>2), no grating lobe will enter visible space independent of scan angle, since |sin(θ)|≦1.
Radiating elements placed in an equilateral triangular grid are shown in FIG. 4. The vertical element spacing is defined as dy. A corresponding beam space is illustrated in FIG. 5. The element spacing must not be greater than 1/√{square root over (3)} wavelengths (i.e. a maximum value of dy is about 0.58 wavelengths) along the y-axis (and 2dx is one wavelength along the x-axis [equal to dy√{square root over (3)}=0.58·λ·√{square root over (3)}=λ]) to avoid generating grating lobes for any scan angle. Thus the optimum element spacing, dy, in an equilateral triangular grid of radiating elements is 1/√{square root over (3)} wavelengths. For a large array, the number of radiating elements in the triangular arranged grid is approximately given by NT=A/(2dxdy). (Also see reference E. D. Sharp mentioned above.) A reduction of (NR−NT)/NR=13% is obtainable for the equilateral triangular grid compared to the square grid assuming the same grating lobe free scan volume. (NT=4A/λ2 and NR=2A√{square root over (3)}/λ2.)
However there is still a demand for an optimisation of the radiating grid in an array antenna for obtaining a scanning sparse antenna array, which provides a further suppressing of grating lobes within visible space.